FIG. 1 is a block diagram showing a communication system using a submarine cable in a conventional technique. In FIG. 1, an A station 100 and a B station 101, which are land optical transceiver stations, communicate with each other via optical fibers for transmission paths in the submarine cable. Each of the A station 100 and the B station 101 transmits or receives, as a main signal, a wavelength division multiplexing (WDM) signal λw of signals with a plurality of wavelengths different from each other. It is to be noted that a communication between the A station 100 and the B station is a “point-to-point communication”.
The A station 100 can transmit a monitor signal λa as an optical signal used to monitor the transmission path from the A station 100 to the B station 101 (hereinafter, to be referred to as an “A ● B route”) The B station 101 can transmit a monitor signal λb as an optical signal used to monitor the transmission path from the B station 101 to the A station 100 (hereinafter, to be referred to as a “B ● A route”). If a distance between the A station 100 and the B station 101 is long, repeaters for relaying the communication are arranged on the transmission paths between the A station 100 and the B station 101. The repeater includes an optical loopback circuit 1a, 2a, . . . , or na for looping back the monitor signals λa and λb.
The monitor signal λa transmitted from the A station 100 to the A ● B route passes through the respective optical loopback circuits 1a, 2a, . . . , and na and arrives at the B station 101. Furthermore, each of the optical loopback circuits 1a, 2a, . . . , and na separates the monitor signal λa in a predetermined intensity ratio and loops back the monitor signal λa to the B ● A route. Each of the optical loopback circuits 1a, 2a, . . . , and na can also loop back reflected light and scattered light generated on the transmission path. It should be noted that the monitor signal λb transmitted from the B station 101 is looped back to the B station 101 by the similar process performed on the monitor signal λa. The A station 100 and the B station 101 receive the monitor signals λa and λb looped back via the transmission paths, respectively.
Each of the A station 100 and the B station 101 analyzes an optical intensity level of the received monitor signal λa or λb, time from transmission to reception of the monitor signal λa or λb, signals superimposed on the received monitor signal λa or λb, and the like, and monitors failures such as a reduction in a level of the repeater, an increase in an optical loss of the optical fiber and disconnection of the optical fiber. Each of the A station 100 and the B station 101 monitors whether or not the failure has occurred, a type of the failure and a location of occurrence of the failure. Furthermore, each of the A station 100 and the B station 101 can monitor a relation between an optical loss amount and a distance in detail by analyzing the reflected light and the scattered light in a transmission direction of the monitor signal λa or λb by using optical time domain reflectometry (OTDR).
Recently, a demand has risen for an adding/dropping technique to add or drop an optical signal to or from a WDM signal transmitted via an optical fiber in a submarine cable. If the adding/dropping is performed, it is necessary to monitor an add/drop transmission path for the added optical signal or dropped optical signal.
Examples of a technique for monitoring such an add/drop transmission path are known as a line monitoring apparatus disclosed in Japanese Patent Application Publication (JP-A-Heisei 9-289494: related art 1) and an optical line monitoring system disclosed in Japanese Patent Application Publication (JP-A-Heisei 10-256995: related art 2).
In the line monitoring apparatus disclosed in the related art 1, a waveform separating unit for adding or dropping an optical signal to or from a main signal is provided between a transmission side station and a reception side station. Furthermore, an add/drop station is provided in the line monitoring apparatus to transmit or receive the added or dropped optical signal. Moreover, an optical loopback circuit is provided on an add/drop transmission path between the wavelength separating unit and the add/drop station. The wavelength separating unit drops not only the main signal but also a monitor signal having a preset wavelength to the add/drop transmission path. The optical loopback circuit loops back the dropped monitor signal to the transmission side station that transmitted the monitor signal. The transmission side station receives the looped-back monitor signal, thereby making it possible to monitor the add/drop transmission path.
Moreover, in the optical line monitoring system disclosed in the related art 2, add/drop stations are provided between trunk stations that serve as transmission or reception side stations, to add or drop a wavelength signal to or from a main signal. Optical loopback circuits are provided between the trunk stations. At least one of the trunk stations and the add/drop stations is used as a monitoring station. A monitor signal transmitted from the monitoring station passes through the add/drop stations other than the monitoring station. The optical loopback circuit loops back the monitor signal to the monitoring station. The monitoring station receives the looped-back monitor signal and can thereby monitor a transmission path on which the monitor signal is transmitted.
In the optical line monitoring system disclosed in the related art 2 of the conventional examples, if one of the trunk stations is used as the monitoring station, then the monitor signal passes through the drop stations. Therefore, the monitor signal is not transmitted onto an add/drop transmission path. Thus, if one of the trunk stations is used as the monitoring station, the add/drop transmission path cannot be monitored. In order to monitor the add/drop transmission path, it is necessary to use one of the drop stations as the monitoring station.
The line monitoring apparatus disclosed in the related art 1 has the following problems. When a plurality of wavelength add/drop units are used, it is required to set wavelengths to be added or dropped for the respective wavelength separating units. Thus, as the number of wavelength add/drop units increases, the number of wavelengths used for monitor signals increases and a wavelength bandwidth available for a main signal decreases accordingly.
The optical line monitoring system disclosed in the related art 2 has the following problems. When a plurality of wavelength add/drop stations are present, an add/drop transmission path is provided for each add-drop station to transmit a signal to be added or dropped in each of the add/drop stations. In order to monitor each add/drop transmission path, it is necessary to use each add/drop station as the monitoring station.
Since the monitor signal passes through the add/drop stations other than the monitoring station that transmitted the monitor signal, the monitor signal transmitted from the monitoring station is not transmitted onto the add/drop transmission path provided in the add/drop stations other than the monitoring station. Thus, in order to monitor all the add/drop transmission paths, all the add/drop stations should be used as the monitoring stations. Further, the monitor signals transmitted from the monitoring stations must differ from each other in wavelength so that the monitoring station can pass through the monitor signals transmitted from the other monitoring stations.
As a result, similarly to the line monitoring apparatus disclosed in the related art 1, the optical line monitoring system disclosed in the related art 2 has the problems that the number of wavelengths used for the monitor signals increases as the number of the add/drop stations increases and that a wavelength bandwidth available for a main signal decreases.